1. Technical Field of the Invention
The present invention relates to reversibly variable electrochromic devices for varying the transmittance to light, such as electrochromic rearview mirrors, windows and sun roofs for motor vehicles, reversibly variable electrochromic elements therefor and processes for making such devices and elements.
2. Brief Description of the Related Technology
Reversibly variable electrochromic devices are known in the art. In such devices, the intensity of light (e.g., visible, infrared, ultraviolet or other distinct or overlapping electromagnetic radiation) is modulated by passing the light through an electrochromic medium. The electrochromic medium is disposed between two conductive electrodes, at least one of which is typically transparent, which causes the medium to undergo reversible electrochemical reactions when potential differences are applied across the two electrodes. Some examples of these prior art devices are described in U.S. Pat. No. 3,280,701 (Donnelly); U.S. Pat. No. 3,451,741 (Manos); U.S. Pat. No. 3,806,229 (Schoot); U.S. Pat. No. 4,712,879 (Lynam) ("Lynam I); U.S. Pat. No. 4,902,108 (Byker) ("Byker I"); and I. F. Chang, "Electrochromic and Electrochemichromic Materials and Phenomena", in Nonemissive Electrooptic Displays, 155-96, A. R. Kmetz and F. K. von Willisen, eds., Plenum Press, New York (1976).
Reversibly variable electrochromic media include those wherein the electrochemical reaction takes place in a solid film or occurs entirely in a liquid solution. See e.g., Chang.
Numerous devices using an electrochromic medium, wherein the electrochemical reaction takes place entirely in a solution, are known in the art. Some examples are described in U.S. Pat. No. 3,453,038 (Kissa); U.S. Pat. No. 5,128,799 (Byker) ("Byker II"); Donnelly; Manos; Schoot; Bvker I; and commonly assigned U.S. Pat. No. 5,073,012 (Lynam) ("Lynam II"); U.S. Pat. No. 5,115,346 (Lynam) ("Lynam III"); U.S. Pat. No. 5,140,455 (Varaprasad) ("Varaprasad I"); U.S. Pat. No. 5,142,407 (Varaprasad) ("Varaprasad II"); U.S. Pat. No. 5,151,816 (Varaprasad) ("Varaprasad III") and U.S. Pat. No. 5,239,405 (Varaprasad) ("Varaprasad IV"); and commonly assigned co-pending U.S. patent application Ser. No. 07/935,784 (filed Aug. 27, 1992)]. Typically, these electrochromic devices, sometimes referred to as electrochemichromic devices, are single-compartment, self-erasing, solution-phase electrochromic devices. See e.g., Manos, Byker I and Byker II.
In single-compartment, self-erasing, solution-phase electrochromic devices, the intensity of the electromagnetic radiation is modulated by passing through a solution held in a compartment. The solution often includes a solvent, at least one anodic compound and at least one cathodic compound. During operation of such devices, the solution is fluid, although it may be gelled or made highly viscous with a thickening agent, and the solution components, including the anodic compounds and cathodic compounds, do not precipitate. See e.g., Byker I and Byker II.
Certain of these electrochemichromic devices have presented drawbacks. First, a susceptibility exists for distinct bands of color to form adjacent the bus bars after having retained a colored state over a prolonged period of time. This undesirable event is known as segregation. Second, processing and manufacturing limitations are presented with electrochemichromic devices containing electrochemichromic solutions. For instance, in the case of electrochemichromic devices which contain an electrochemichromic solution within a compartment or cavity thereof, the size and shape of the electrochemichromic device is limited by the bulges and non-uniformities which often form in such large area electrochemichromic devices because of the hydrostatic nature of the liquid solution. Third, from a safety standpoint, in the event an electrochemichromic device should break or become damaged through fracture or rupture, it is important for the device to maintain its integrity so that, if the substrates of the device are shattered, an electrochemichromic solution does not escape therefrom and that shards of glass and the like are retained and do not scatter about. In the known electrochromic devices, measures to reduce breakage or broken glass scattering include the use of tempered glass and/or a laminate assembly comprising at least two panels affixed to one another by an adhesive. Such measures control the scattering of glass shards in the event of breakage or damage due, for instance, to the impact caused by an accident.
Numerous devices using an electrochromic medium, wherein the electrochemical reaction takes place in a solid layer, are known in the art. Typically, these devices employ electrochromic solid-state thin film technology [see e.g., N. R. Lynam, "Electrochromic Automotive Day/Night Mirrors", SAE Technical Paper Series, 870636 (1987); N. R. Lynam, "Smart Windows for Automobiles", SAE Technical Paper Series, 900419 (1990); N. R. Lynam and A. Agrawal, "Automotive Applications of Chromogenic Materials", Large Area Chromogenics: Materials & Devices for Transmittance Control, C. M. Lampert and C. G. Granquist, eds., Optical Eng'g Press, Washington (1990); C.M. Lampert, "Electrochromic Devices and Devices for Energy Efficient Windows", Solar Energy Materials, 11, 1-27 (1984); U.S. Pat. No. 3,521,941 (Deb); U.S. Pat. No. 4,174,152 (Giglia); U.S. Pat. No. Re. 30,835 (Giglia); U.S. Pat. No. 4,338,000 (Kamimori); U.S. Pat. No. 4,652,090 (Uchikawa); U.S. Pat. No. 4,671,619 (Kamimori); Lynam I; and commonly assigned U.S. Pat. No. 5,066,112 (Lynam) ("Lynam IV") and U.S. Pat. No. 5,076,674 (Lynam) ("Lynam V")].
In solid-state thin film electrochromic devices, an anodic electrochromic layer and a cathodic electrochromic layer, each layer usually made from inorganic metal oxides, are typically separate and distinct from one another and assembled in a spaced-apart relationship. The solid-state thin films are often formed using techniques such as chemical vapor deposition or physical vapor deposition. Such techniques are not attractive economically, however, as they involve cost. In another type of solid-state thin film electrochromic device, two substrates are coated separately with compositions of photo- or thermo-setting monomers or oligomers to form on one of the substrates an electrochromic layer, with the electrochromic material present within the layer being predominantly an inorganic material, and on the other substrate a redox layer. [See Japanese Patent Document JP 63-262,624].
Attempts have been made to prepare electrochromic media from polymers. For example, it has been reported that electrochromic polymer layers may be prepared by dissolving in a solvent organic polymers, which contain no functionality capable of further polymerization, together with an electrochromic compound, and thereafter casting or coating the resulting solution onto an electrode. It has been reported further that electrochromic polymer layers are created upon evaporation of the solvent by pressure reduction and/or temperature elevation. [See e.g., U.S. Pat. No. 3,652,149 (Rogers), U.S. Pat. No. 3,774,988 (Rogers) and U.S. Pat. No. 3,873,185 (Rogers); U.S. Pat. No. 4,550,982 (Hirai); Japanese Patent Document JP 52-10,745; and Y. Hirai and C. Tani, "Electrochromism for Organic Materials in Polymeric All-Solid State Systems", Appl. Phys. Lett., 43(7), 704-05 (1983)]. Use of such polymer solution casting systems has disadvantages, however, including the need to evaporate the solvent prior to assembling devices to form polymer electrochromic layers. This additional processing step adds to the cost of manufacture through increased capital expenditures and energy requirements, involves potential exposure to hazardous chemical vapors and constrains the type of device to be manufactured.
A thermally cured polymer gel film containing a single organic electrochromic compound has also been reported for use in display devices. [See H. Tsutsumi et al., "Polymer Gel Films with Simple Organic Electrochromics for Single-Film Electrochromic Devices", J. Polym. Sci., 30, 1725-29 (1992) and H. Tsutsumi et al., "Single Polymer Gel Film Electrochromic Device", Electrochemica Acta, 37, 369-70 (1992)]. The gel film reported therein was said to possess a solvent-like environment around the electrochromic compounds of that film. This gel film was reported to turn brown, and ceased to perform color-bleach cycles, after only 35,200 color-bleach cycles.